Stimulated Raman scattering of CO.sub.2 laser radiation in parahydrogen has been suggested by others as a means of generating efficient powerful radiation in the 16 micron region of the electromagnetic spectrum, a region of intense interest because of its powerful usefulness for uranium isotope separation employing UF.sub.6.
While the generation of radiation by Raman scattering in many ways is similar to the generation and amplification of radiation by conventional laser techniques, the differences between the physics of laser amplification and stimulated Raman scattering present different problems which must be dealt with in different ways. Thus, in a laser device, the medium is pumped to a state in which amplification may occur for a period of time after such state is achieved. The energization of the medium is an independent process from the amplification by stimulated emission. In stimulated Raman scattering, on the other hand, the medium is energized by a beam of radiation only while that beam is present. Thus, in order to amplify by stimulated Raman scattering, it is necessary to have a pumping light beam follow the same path during the same time as the beam being amplified.
It is also known that the gain per unit length achievable by stimulated Raman scattering at a point is a function of the intensity of the pump radiation at that point. It is also known that even the most collimated of light beams spread by diffraction. Thus, since the gain per unit length achievable by stimulated Raman scattering is a function of intensity, the gain drops as the pump radiation beam spreads lowering the intensity at any point therein. Thus, in order to maximize gain achievable by stimulated Raman scattering along a predetermined path length, it is desirable to repeatedly refocus the beam to achieve the higher intensities resulting in the higher gains.
One possibility that we examined to maintain high intensity along a path was the use of an optical waveguide to confine a tightly focussed beam to effectively extend the focal zone through the length of the guide thereby completely eliminating the effect of diffraction. The problem with a waveguide scheme is that the material of the guide is in close proximity to the intense optical field and damages easily, promoting plasma breakdown at much lower intensities than in an unconfined medium. Work relating to this scheme was published more than a year ago (September 1976) in "Applied Optics", Vol. 15, No. 9, Page 2005, in an article entitled "Waveguide H.sub.2 Raman Laser."
Another possibility that was considered for the purpose of reestablishing high intensities sequentially along a gain path was the use of a periodic sequence of lenses to sequentially refocus the pumping light beam, thereby building up gain between each pair of lenses. Several difficulties arise from such a scheme. The most important difficulty is that the dispersion in any material that would cause sequential focussing of the pump radiation would have a different effect upon the stimulated radiation thereby focussing the pump radiation and the stimulated radiation in different places along the path, thereby reducing the total gain because of the fact that the radiation to be amplified would not be focussed in the same place as the pumping radiation. Further, this mismatch of focussing would be cumulative.
Many devices can be conceived of which include resonant structures for 16 microns which would provide sufficiently intense 16 micron radiation by rotational Raman scattering in parahydrogen if the source of pumping radiation could produce pumping radiation of sufficient power for long periods of time. Unfortunately, however, under the present state of the art, a pulse of approximately 100 nano-seconds in length is the maximum that is available to provide the pumping radiation. Thus, someone trying to produce 16 micron radiation by stimulated rotational Raman scattering in parahydrogen has this limitation to face as well.